What MAC sizes are there?

MAC-48 or EUI-48: 48 bits: 01:41:93:AB:CD:EF

EUI-64: 64 bits: 01:41:93:FF:FF:AB:CD:EF

The EUI-64 numbering system encompasses both MAC-48 and EUI-48 identifiers by a simple translation mechanism. To
convert a MAC-48 into an EUI-64, copy the OUI, append the two octets FF-FF and then copy the
organization-specified extension identifier.

The same procedure is used to convert an EUI-48 into an EUI-64, but the sequence inserted will then be FF-FE. In
both cases, the process could be trivially reversed when necessary.

Organizations issuing EUI-64s are cautioned against issuing identifiers that could be confused with these forms.
The IEEE policy is to discourage new uses of 48-bit identifiers in favor of the EUI-64 system.

IPv6 — one of the most prominent standards that uses a Modified EUI-64 — treats MAC-48 as EUI-48 instead (as it
is chosen from the same address pool) and toggles the U/L bit (as this makes it easier to type locally assigned
IPv6 addresses based on the Modified EUI-64). This results in extending MAC addresses (such as IEEE 802 MAC
address) to Modified EUI-64 using only FF-FE (and never FF-FF) and with the U/L bit inverted.

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